The present invention relates to a ceramic Composite material, for example, a ceramic molded body or a layer, as well as a use of the ceramic molded body or a layer.
European Published Patent Application No. 0 412 428 refers to a ceramic composite body and a method of producing same, in which an organosilicon polymer, as a precursor material, together with incorporated particles of hard material and/or other reinforcing components, as well as one or more metallic fillers, is subjected to pyrolysis. In pyrolysis, the decomposition products formed from the polymer compounds react with the metallic filler, which may result in a ceramic composite body having a matrix with particles of hard material and/or reenforce embedded components.
For example, carbides or nitrides of titanium, zirconium or other transition metals may be used as the hard material particles or reinforcing components as referred to, for example, in European Published Patent Application No. 0 412 428, in which the particle sizes of the powder particles are in the range of approximately 1 xcexcm to approximately 300 xcexcm.
The matrix formed from the organosilicon polymer after pyrolysis is a monophase or polyphase, amorphous, partially crystalline or crystalline matrix of silicon carbide, silicon nitride, silicon dioxide or mixtures thereof.
In addition to microscale powder materials, nanoscale powder materials may be single-phase or polyphase powders having particle sizes in the nanometer range. Due to their small particle dimensions, they are characterized by a very high proportion of particle boundaries or phase boundaries per volume. In addition, the physical, chemical and mechanical properties of such nanoscale powders may differ from those of conventional coarse-grained materials having the same chemical composition. For example, such nanoscale powders may have greater hardness, increased diffusivity and increased specific heat.
Nanoscale powdered materials may be produced by flame pyrolysis, gas condensation, spray conversion or crystallization of amorphous substances. Industrial production has advanced in the case of zirconium dioxide, silicon dioxide, titanium dioxide and aluminum oxide.
It is believed that the properties of ceramic composite materials having microscale fillers are determined largely by the properties of the fillers. Thus, local stress peaks or cracks may occur in the composite material when the properties of the matrix and fillers differ, e.g., different coefficients of thermal expansion. This may result in an increased failure rate of such components.
When using reactive microscale fillers as referred to, for example, in European Published Patent Application No. 0 412 428, the effect of which is based on reaction of the fillers with the ambient matrix, only an incomplete reactive conversion of filler may be achieved in the edge area of the filler grains.
An object of the present invention is to provide a ceramic composite material, which may be suitable, for example, for producing ceramic molded bodies or layers and with which the profile of electrical and physical properties may be easily and reliably adjusted.
Another object of the present invention is to provide a ceramic composite material, the electric properties of which, porosity, high-temperature stability, mechanical strength, i.e., fracture toughness and homogeneity are improved in comparison with the related art.
It is believed that an exemplary ceramic composite material according to the present invention has the advantage in that the profile of electrical and physical properties of the ceramic composite material obtained after pyrolysis may be adapted to a profile of properties predetermined for the respective application, i.e., the composition of the composite material may be tailored to this profile of properties. For example, the large selection of fillers may permit the properties of the resulting ceramic composite materials to be varied or adjusted over a wide spectrum.
In addition, it is believed that an exemplary ceramic composite material according to the present invention has the advantage in that, due to the small particle size of the reactive filler, the process temperatures may be lowered and the process times required for a complete reaction may be shortened in comparison with the related art, so that with the process temperatures required in the past, liquid or volatile fillers may still be solid and thus may be used at the pyrolysis and sintering temperatures. Furthermore, unwanted phase reactions, which may occur at higher temperatures, i.e., reactions between the matrix and filler, may be avoided by using reduced process temperatures.
It is believed that one advantage of the composite material according to the present invention is that the porosity of the composite material may be adjusted in a defined manner using the fillers, the combination of a suitable nanoscale filler with defined pyrolysis conditions thus allowing the production of both highly porous composite materials and dense composite materials by varying the pyrolysis conditions, while otherwise using the same polymer precursor material, i.e., the same starting mixture.
Exemplary porous ceramic composite materials according to the present invention may also have a very good spalling resistance and may be applied to various applications, for example, as lightweight structural materials, as porous protective shells for sensors, as filters, as catalyst support materials or as a matrix for infiltrated reactive composite materials, while exemplary high-density ceramic composite materials according to the present invention have an increased mechanical strength, improved fracture toughness and improved corrosion resistance.
In production of a n exemplary ceramic composite material according to the present invention, shaping and production methods may be used, so that even ceramic fibers, layers and molded bodies of different sizes or having a complex geometry are readily obtainable, which may permit an exemplary composite material to be applied to a broad spectrum of applications. For example, shaping methods, such as compression molding, injection molding, joining and fiber extrusion may be used. With regard to the production method used, pyrolysis, under a protective gas and laser pyrolysis may be employed.
In this regard, a simple and reliable control or adjustability of the flow properties and pourability of the starting mixture may be achieved through the type and quantity of the nanoscale filler. This may also be true of the process parameters in powder transport, in cold molding, in injection molding, in spin coating or in dip coating.
Moreover, due to the small size of the filler, detailed replicas of embossed, cast or injection molded shapes may also be produced by pouring the starting mixture into a mold and then performing pyrolysis. In addition to the fidelity in detail, these replicas may have a high surface quality, allowing details having dimensions of less than 1 xcexcm to be molded.
It is also believed that an exemplary ceramic composite material according to the present invention has the advantage in that, due to the use of highly dispersed insulating fillers, the electric resistance of the composite material is increased significantly and the long-term stability of this electric resistance may be improved. In addition, due to the improved homogeneity and stability of the thermal and electrical properties of the resulting composite material, reliability may also increase.
It is also believed that another advantage of an exemplary ceramic composite material according to the present invention is that it may permit high degrees of filling and short pyrolysis times, and the flow properties of the polymer precursor materials used may be regulated through the addition of suitably selected fillers. Thus, for example, suspensions of starting mixtures that remain stable and processable over long periods of time may-be produced.
The polymer precursor material may be an oxygen-containing polysiloxane precursor or a polysilazane precursor that is stable in air, since these materials allow processing in air and thus may allow the production of inexpensive composite materials. In addition, the resulting pyrolysis product may be chemically stable with regard to oxidation and corrosion and at the same time may be unobjectionable from a health standpoint.
In addition to the nanoscale fillers having an average particle size of less 200 nm, other fillers, such as a powdered aluminum oxide (Al2O3) having a larger particle size of 500 nm to 10 xcexcm, for example, 500 nm to 3 xcexcm, may also be used. This may broaden the spectrum of achievable electrical and physical properties and thus may broaden the spectrum of applications of the resulting composite materials. For example, the electric resistance of the resulting ceramic composite material may increase by several orders of magnitude at room temperature and also at temperatures greater than 1200xc2x0 C. Also, when conventional microscale aluminum oxide fillers are replaced largely or completely by nanoscale silicon dioxide, for example, amorphous silicon dioxide or corresponding highly dispersed silicic acid, for example, pyrogenic silicic acid, the long-term stability of the mechanical and electrical properties of the ceramic composite material obtained may be improved at temperatures above 1200xc2x0 C. Simultaneously, an increase in the allowed heating rates in pyrolysis and a shortening of the time required for shaping by compression molding may be achieved.
With regard to the highest possible specific electric resistance of the composite material, it is believed to be advantageous if, in addition to the polymer precursor material and instead of or in addition to a conventional, microscale aluminum oxide filler, the starting mixture also contains nanoscale silicon dioxide, for example, amorphous silicon dioxide, nanoscale silicon dioxide provided having a carbonaceous and/or hydrophilic surface modification, pyrogenic silicic acid or silicic acid provided with a carbonaceous and/or hydrophilic surface modification to which may be added a boron compound in the amount of 10 wt % to 30 wt %, for example, a boron oxide such as B2O3.
In this connection, the specific electric resistance of the resulting composite material depends not only on the particle size of the filler but also on the BET surface area of the filler, so that the resistance may be easily adjustable to unexpectedly high values. The surface properties of the filler are additional variables, which effect the resulting specific electric resistance of the composite material, for example, in conjunction with a change in the BET surface area. Thus, the transition from a hydrophobic surface to a hydrophilic surface of the filler particles, for example, may result in an increase in the specific electric resistance obtained.
Especially high values for the specific electric resistance may also be achieved, for example, when the filler, for example, SiO2 or silicic acid, is used in an amount of at least 9 vol % in the starting mixture, whereby at the same time, another filler such as Al2O3, which may optionally be used in the starting mixture, should amount to less than 7 vol %, for example, less than 3 vol %.
Due to the small particle size of the filler, the surface quality of coatings produced with this ceramic composite material may be improved because the starting mixture applied before pyrolysis to the surface of a substrate to be coated penetrates into all the surface detects and irregularities in this substrate, thereby increasing adhesion of the coating, as well as equalizing irregularities and defects in the substrate-layer interface.
With respect to nanoscale fillers, at least approximately complete conversion of these fillers with the surrounding matrix in pyrolysis may be achieved in the ceramic composite material. This may result in, for example, a definite shortening of pyrolysis cycles. Furthermore, chemical reaction of the nanoscale filler with the polymer precursor material may proceed more rapidly in comparison with microscale fillers.
Also, by adding a suitable stabilizer to the starting mixture, for example, production of a stable suspension of the polymer precursor material with the filler in an organic solvent may be produced. For example, the stability of such a suspension with respect to sedimentation may increases in comparison with similar starting mixtures having microscale fillers, so that coating methods performed with such suspensions by dip coating or spin coating may be facilitated. Furthermore, the nanoscale filler may be suitable as a dispersant for a microscale filler used concurrently.